The commonly used temperature sensors include thermocouples, resistive temperature devices (RTDs), thermistors, diode sensors, and transistor temperature sensors. The common form of these temperature sensors is a sensing element with extension wires connecting the sensing element to a remote temperature reading device. The sensing element can be a hot junction of two dissimilar metals, an RTD element, a thermistor bead, a diode, or a transistor. The electrical resistance or electrical output or characteristic of the sensing element changes with temperature. Determining the change in the electrical characteristics of the sensing element results in the determination of the temperature of the sensing element, and hence the temperature of an object in the vicinity of the sensing element. Due to the delicacy of the sensing element and the extension wires, a metal tube or a sheath generally surrounds the extension wires and the sensing element and an insulating material is provided to protect the wires and sensing element.
Various attempts have been made to provide a robust configuration for temperature sensors. One common method of manufacturing a sensor is to encase the extension wires and the sensing element in a tube and fill the tube with an insulating powder. The insulating powder is then fired to form a hard and brittle insulator. However, this method produces a sensor that has a hard brittle ceramic insulator that cannot be bent and is susceptible to damage from vibration or impact.
Another common method of manufacturing a sensing element is to provide a preformed crushable ceramic insulator around the extension wires before the tube is assembled. The extension wires are placed inside the preformed crushable insulator and the package is placed into a tube with the sensing element being located near an end of the tube that may have been crimped or otherwise narrowed. The sensing element is not surrounded by insulating material. Thereafter, the main body portion is swaged to reduce the outer diameter of the main body portion thereby compacting the crushable ceramic insulator into a crushed powder around the extension wires. As a result, the extension wires are held in place by the crushed and compacted ceramic powder with the sensing element remaining in an open un-insulated inner space defined by the narrowed portion. After the preformed insulator is crushed, a ceramic insulating powder is introduced through the opened narrowed end of the tube and packed around the sensing element. The opening of the tube is then closed.
Another common method of manufacturing a temperature sensor is to form the leadwire portion separate from the sensing element by providing a preformed crushable ceramic insulator around the extension wires before the tube is assembled. The extension wires are placed inside the preformed crushable insulator and the package is placed into a tube. Thereafter, the main body portion is swaged to reduce the outer diameter of the main body portion thereby compacting the crushable ceramic insulator into a crushed powder around the extension wires. As a result, the extension wires are held in place by the crushed and compacted ceramic powder with the sensing element remaining in an open un-insulated inner space defined by the narrowed portion. After the preformed insulator is crushed, a cavity in the crushed insulation is formed to accommodate the introduction of the sensing element. After the sensing element is attached to the leadwire in the cavity the remaining space in the cavity is filled with insulating powder and the tube is capped.
These methods of manufacturing a temperature sensor do not produce a robust structure for the temperature sensor and in particular the sensing element. The ceramic insulating powder is loosely distributed around the sensing element and often does not provide sufficient protection for the sensing element against vibration or shock. Moreover, this method requires many separate and costly manufacturing steps.